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d-and f-Block Elements - The Actinoids

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The actinoids include the 14 elements from Thorium (Z=90Z=90) to Lawrencium (Z=103Z=103). They involve the filling of the 5f5f orbital.

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General electronic configuration is [Rn]5f0−146d0−17s2[Rn] 5f^{0-14} 6d^{0-1} 7s^2. The 5f5f and 6d6d orbitals are close in energy, leading to variations in the filling of the subshells.

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Actinoids exhibit a larger variety of oxidation states (up to +7+7) compared to lanthanoids because the 5f,6d5f, 6d, and 7s7s energy levels are comparable, allowing more electrons to participate in bond formation.

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Actinoid Contraction: There is a gradual decrease in the size of atoms or M3+M^{3+} ions across the series. This contraction is greater than lanthanoid contraction due to the poorer shielding effect of 5f5f electrons compared to 4f4f electrons.

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All actinoids are radioactive metals. Elements beyond Uranium (Z=92Z=92) are synthetic and known as transuranic elements.

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Actinoids are more reactive than lanthanoids. They react with boiling water to give a mixture of oxide and hydride, and they combine with most non-metals at moderate temperatures.

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Most M3+M^{3+} and M4+M^{4+} actinoid ions are colored due to f−ff-f transitions. For example, U3+U^{3+} is red and U4+U^{4+} is green.

📐Formulae

[Rn]5f1−146d0−17s2[Rn] 5f^{1-14} 6d^{0-1} 7s^2

Ionic Radius: M3+>M4+\text{Ionic Radius: } M^{3+} > M^{4+}

Common Oxidation State: +3\text{Common Oxidation State: } +3

Max Oxidation States: Np(+7),Pu(+7),U(+6)\text{Max Oxidation States: } Np (+7), Pu (+7), U (+6)

💡Examples

Problem 1:

Explain why actinoids show a wider range of oxidation states than lanthanoids.

Solution:

This is because the energy gap between 5f,6d5f, 6d, and 7s7s subshells in actinoids is very small.

Explanation:

In lanthanoids, the energy of 4f4f is much lower than 5d5d and 6s6s. In actinoids, the 5f5f electrons are less deeply buried, so 5f,6d5f, 6d, and 7s7s electrons can all participate in chemical bonding, resulting in oxidation states like +4,+5,+6,+4, +5, +6, and +7+7.

Problem 2:

Compare the magnetic properties of actinoids with lanthanoids.

Solution:

Actinoids show more complex magnetic behavior and generally higher paramagnetism than lanthanoids.

Explanation:

The 5f5f electrons in actinoids are more effectively shielded from the nucleus but less shielded from external fields compared to 4f4f electrons. This results in more varied magnetic susceptibility that is often harder to interpret via the simple spin-only formula.

Problem 3:

Why is actinoid contraction greater than lanthanoid contraction?

Solution:

Due to the poorer shielding effect of 5f5f electrons compared to 4f4f electrons.

Explanation:

As the nuclear charge increases across the series, the 5f5f electrons are added. Because 5f5f orbitals are more diffused than 4f4f orbitals, they provide less effective shielding of the outer electrons from the increasing nuclear charge, leading to a more pronounced decrease in size.

The Actinoids - Revision Notes & Key Formulas | CBSE Class 12 Chemistry